Crude oil typically contains several hundred thousand compounds. A common simplified analysis system classifies these compounds into four groups according to their solubility. This system is known as “SARA” analysis, where the groups are saturates, aromatics, resins, and asphaltenes. The least soluble of these groups are the asphaltenes, which can be stabilized by association with resins and/or aromatics but are destabilized by association with saturates. Asphaltenes typically exist as nano-scale stabilized dispersion in the resin, aromatics, and saturates mix. If the balance of these components is disturbed, as it can be during thermal cracking, conditions may arise where asphaltenes precipitate from solution. Among other problems, this thermal cracking can lead to coke formation at high temperatures and sludge in visbreaker tar residue.
Asphaltenes are of particular interest to the petroleum industry because of their depositional effect in production equipment. Asphaltenes also impart high viscosity to crude oils, negatively impacting production. Variable asphaltene concentration in crude oils within individual reservoirs creates a myriad of production problems. Refining of heavier crudes poses problems to petroleum producers and refiners. During production, unwanted asphaltene precipitation causes well plugging. During refining, asphaltenes cause refinery heat exchanger fouling, as well as catalyst poisoning by coking or binding of active sites with heavy metals.
Asphaltenes in crude oil, fuel oil, distillation residue, and the like are insoluble in heptane at its boiling point and soluble in benzene at its boiling point. They are typically black to dark brown solids having a molecular structure of polynuclear aromatic rings with alkyl side chains and heteroatoms, such as nitrogen, oxygen, and sulfur. These solubility characteristics allow its indirect measurement.
For example, U.S. Pat. No. 4,940,900 to Lambert discloses measurement of the flocculation threshold of a petroleum product containing asphaltenes by continuously adding a precipitant and measuring the near infrared radiation transmitted through a sample of the product in relation to the quantity of added precipitant. The method requires addition of both a solvent and precipitant to the asphaltene-containing product tested. U.S. Pat. No. 5,452,232 to Espinosa et al. discloses a method of determining properties and yield of a hydrocarbon conversion product from the NIR spectrum of the feedstock. Mid infrared has also been used to determine the functional groups in asphaltenes by methyl ratio, paraffinic and naphthenic carbons, and alkyl side chain length.
Current practice for determining the asphaltene stability of hydrocarbon process streams involves using some form of a heptane phase separation method. In that method, heptane is added to a sample from a hydrocarbon process stream, which dilutes the sample and decreases its absorbance. At the end point, the absorbance begins to increase due to asphaltene (or other condensed aromatic compound) precipitation. Asphaltene stability is than calculated based upon the absorbance readings. A typical method of this type is described in detail in “Standard Test Method for Determination of Intrinsic Stability of Asphaltene-Containing Residues, Heavy Fuel Oils, and Crude Oils (n-Heptane Phase Separation; Optical Detection), published by ASTM International in May 2005 under “Designation D7157-05).” In addition to being time-consuming, a disadvantage of this method is that it requires multiple dilutions of the sample, each then being titrated with the n-heptane solvent for evaluation. These methods also significantly limit the ability to optimize the cracking process, especially under conditions of frequent changes in the type of crude oil in the feedstream.
There thus exists an ongoing need for improved methods of determining stability in hydrocarbon process streams. A particular need exists for quickly and efficiently determining process stream stability with frequent changes in feed type.